The latter half of the twentieth century has been witness to a phenomenon known as the information revolution. While the information revolution is a historical development broader in scope than any one event or machine, no single device has come to represent the information revolution more than the digital electronic computer. The development of computer systems has surely been a revolution. Each year, computer systems grow faster, store more data, and provide more applications to their users.
The extensive data storage needs of modem computer systems require large capacity mass data storage devices. While various data storage technologies are available, the rotating magnetic rigid disk drive has become by far the most ubiquitous. Such a disk drive data storage device is an extremely complex piece of machinery, containing precision mechanical parts, ultra-smooth disk surfaces, high-density magnetically encoded data, and sophisticated electronics for encoding/decoding data, and controlling drive operation. Each disk drive is therefore a miniature world unto itself, containing multiple systems and subsystem, each one of which is needed for proper drive operation. Despite this complexity, rotating magnetic disk drives have a proven record of capacity, performance and cost which make them the storage device of choice for a large variety of applications.
A disk drive typically contains one or more disks attached to a common rotating hub or spindle. Each disk is a thin, flat member having a central aperture for the spindle. Data is recorded on the flat surfaces of the disk, usually on both sides. A transducing head is positioned adjacent the surface of the spinning disk to read and write data. Increased density of data written on the disk surface requires that the transducer be positioned very close to the surface.
The disk is manufactured of a non-magnetic base (substrate), which is coated with a magnetic coating for recording data on the recording surfaces, and which may contain additional layers as well, such as a protective outer coating. Historically, aluminum has been the material of choice for the substrate. In recent years there has been considerable interest in other materials, specifically glass. Ideally, the disk surface is both very flat and very smooth. Deviation in the surface profile from an ideal plane can affect the flying characteristics of the transducer heads, can cause collision or damage to the heads, can affect the data recording characteristics, or have other consequences. Whether the disk substrate is manufactured of aluminum, glass or other material, it is subjected to multiple processing steps to ensure a sufficiently flat, smooth finished disk.
Of course, ideal planes exist only in mathematical theory, and it is impossible to make a physical device which is truly perfectly flat and smooth. Well-known problems of waviness and surface roughness exist. Waviness or warp is a gross deviation from flatness over the entire disk surface. Waviness is generally due to the fact that the disk is readily susceptible to warping as a result of its very thin, annular shape, and various internal stresses are introduced during formation or the many processing steps required to produce an acceptable finished product. A disk transducer head flies a small distance above the surface, and on a warped disk will tend to follow the warp, thus moving up and down as the disk spins under the head. The frequency of the is up and down motion is generally a small multiple (e.g. 2) of the frequency of disk revolution. If the warp is excessive, the head may not be able to follow the disk surface at the prescribed distance. Roughness, on the other is a local surface condition. Excessive roughness may affect the flyheight or the recording characteristics of the disk surface. A typical disk is subjected to one or more polishing steps to reduce roughness.
An intermediate form of surface deviation, herein referred to as “microwaviness”, may exist. As used herein, microwaviness is a waviness of a disk surface for a range of wavelengths that are on the order of the length of the transducing head. Using current head technology, these wavelengths are approximately in the range of 10 to 5000 microns, it being understood that these ranges could change in the future if recording heads change significantly in size.
The significance of microwaviness has not necessarily been appreciated in the past. But as disk drive designs increasingly employ head flying heights closer to the disk surface, microwaviness becomes more significant. For low head flying heights, even small amplitude microwaviness can excite an airbearing resonance, thereby causing excessive head-disk spacing modulation. This modulation can cause poor overwriting of data on the disk surface, which can lead to hard read errors (unrecoverable read errors) as well as an increase in soft read errors (which degrades performance). In some cases, it may even cause collision of the head with the disk surface, potentially damaging the disk surface and/or head, and leading to permanent drive failure in the form of a head crash.
Although not necessarily recognized in the field, it would be desirable to measure the microwaviness characteristics of a disk surface before shipping the finished product to the customer, and preferably before assembling the disk into a disk drive storage device, in order to assure that manufactured disks or disk substrates have sufficiently low amplitude microwaviness so as not to cause problems in the field. Although it is possible to measure disk surface microwaviness using existing laboratory equipment, use of such in a manufacturing environment would be excessively cumbersome and expensive, or have other drawbacks, which would make routine application difficult or impractical. For example, sufficiently accurate measurement of disk surface microwaviness can be achieved with a laboratory instrument called a ZYGO™ optical measurement tool, but using this tool to measure manufactured disks would be impractically time consuming and expensive. A need exists for a faster, practical method of measuring disk microwaviness, and particularly one which could be applied as a spot-screening or an inspection process for disks in a manufacturing environment.